gdb: remove use of a static buffer for building error strings

gdb: remove use of a static buffer for building error strings

I noticed in procfs.c that we use a static buffer for building error
strings when we could easily use std::string and string_printf to
achieve the same result, this commit does that.

I ran into this while performing a further refactor/cleanup that will
be presented in a later patch in this series, and thought this was
worth splitting out into its own patch.

As far as I can tell, only Solaris uses procfs.c, so I did a test
build on a Solaris machine, and I don't believe that I've broken
anything.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>